The present invention relates to a cutting tool for metal machining, comprising at least one body containing CBN, (polycrystalline cubic boron nitride), with or without carbide backing, and a hard and wear resistant refractory coating on the surface of said body. The coating is adherently bonded to the body and covers all functional parts thereof. The coating is composed of one or more layers of refractory compounds of which at least one layer consists of fine-crystalline xcex3-Al2O3, deposited by Physical Vapour Deposition (PVD).
Cutting tools having cutting edges formed of a superhard abrasive such as a cubic boron nitride (CBN) based material are manufactured by powder metallurgical techniques and are mainly used for the machining of cast iron and hardened steel. Several types of CBN cutting tools are known, the majority consisting of a CBN tip that has been brazed onto a carbide insert. Others have the CBN sintered directly to a carbide backing of sufficient thickness to produce an insert while yet others consist of a CBN-containing body without any carbide backing.
Subjecting a sintered CBN body to temperatures over 1000xc2x0 C. may result in unwanted structural changes in the material. Furthermore, in the case of a brazed insert the braze joint will be destroyed.
Swedish patent application 9704066-1 discloses a coated cutting tool for metal machining. The coating is composed of one or more layers of refractory compounds of which at least one layer consists of fine-grained, crystalline xcex3-phase alumina (Al2O3) with a grainsize less than 0.1 xcexcm. The Al2O3-layer is deposited with a bipolar pulsed DMS technique (Dual Magnetron Sputtering) at substrate temperatures in the range 450xc2x0 C. to 700xc2x0 C.
WO 98/28464 discloses that by applying a coating including a MTCVD-TiCN and a CVD-Al2O3-layer to such a CBN tool material, substantial advantages in tool life can be achieved when machining hardened ball bearing steel.
U.S. Pat. No. 5,503,913 proposes improving the wear properties of tools with a cutting edge of cubic boron nitride or polycrystalline cubic boron nitride to coat the superhard body with a 0.5-6 xcexcm thick coating of one or more oxides of the metals Zr, Y, Mg, Ti or Al. The coating is deposited from the gas phase at temperatures up to 800xc2x0 C., preferably using a pulse plasma CVD-process.
According to one aspect of the present invention, there is provided a cutting tool comprising a substrate and a coating, said substrate comprising a body containing at least 35 vol-% cubic boron nitride, and said coating comprising at least one layer of alumina, said alumina layer consisting essentially of xcex3-Al2O3 with a grain size less than 0.1 xcexcm and being free from halogen impurities.
According to another aspect of the present invention, there is provided a method of making a cutting tool, the cutting tool comprising a substrate and a refractory layer of fine-grained crystalline xcex3-Al2O3, the method comprising the steps of: (i) heating the substrate to a temperature of 450-700xc2x0 C.; and (ii) depositing the xcex3-Al2O3 onto the substrate by reactive pulsed magnetron sputtering in a gas mixture of argon and oxygen at a pressure of 1-5 xcexcbar, wherein the pulse frequency is 10-100 kHz, the coating is deposited at a rate of at least 1 nm/s, and the magnetron power density in time average is set for at least 10 W/cm2.